It's not as well plotted or as touching as Galaxy Quest, so it's not as good a film, but then GQ sets a bloody high standard and this one is funny and satirical.
There's satire of the Hollywood system, and straight pastiche of particular war films: Platoon, River Kwai, Apocalypse Now, Thin Red Line and so on. There's some clever stuff for film buffs, like changing the visual production to match the film they are pastiching. There is a pretty amazing performance by Tom Cruise as the head of the film studio.
If you like war films, or you like Hollywood satire, you'll enjoy this. I laughed a lot while I was watching it, not a huge amount stayed with me after.
ETA Review in the Observer (which is what tipped me off that the casting is a spoiler).
EATA: begins with a spoof trailer for a film with Robert Downey Jnr and Toby Maguire which ought to exist